Reproduction at consumers of broadband sounds like music, and broadcast radio and TV, can be done in very high quality using modern storage or transmission media and the appropriate playback equipment, including a proper loudspeaker setup and a suitable, quiet, listening room. In practice, these conditions are not always fulfilled, either due to limitations in the equipment or due to external disturbing or limiting factors. The present invention addresses some of the problems and challenges of sound reproduction in general, but especially those concerning reproduction through small devices with small loudspeakers.
Sound reproduction devices with small loudspeakers and possibly small physical dimensions often suffer from a reduced sound quality compared to a traditional Hi-Fi system. There are several reasons for this, as explained in the following.
First of all, a definition of “small devices” is appropriate. Some examples:                Mobile telephone size: Very small indeed, both in stereo base and loudspeaker size. These devices are typically battery-powered.        Portable CD-radio, iPod speaker systems: Small, both in stereo base and loudspeaker size. These devices are typically powered from the mains.        Flat panel TV, 20-40″ diagonal: Medium-sized and with a stereo base smaller than normal Hi-Fi system and with small space for loudspeakers. These devices are typically mains-powered.        
According to this definition, the present invention addresses primarily sound reproduction through loud-speakers in small and medium-sized devices.
The term “sound quality” consists of several factors, some of which are treated here. Among the factors influencing the sound quality are:                Properties of the signal itself        Transmission and storage media.        Physical/acoustical factors.        External factors. For example, playback conditions may be less than optimal: During transportation, in the kitchen, at night time with soft listening level, etc.        Perceptual factors.        
The present invention focuses on the following aspects of the reproduction:                Bass        Tonal balance at all levels        
To a lesser degree also the following aspects are treated:                Dynamic range        Loudness uniformity        
Various methods to improve the reproduction of bass from small loudspeakers exist. Some earlier methods to improve the bass reproduction from small loudspeakers by employing variable filters include the U.S. Pat. No. 5,481,617 (Bjerre, of Bang and Olufsen A/S) and the US patent application US 2004/0022400 (Magrath).
The method described by Bjerre includes a variable band-pass filter and a compensating differentiator. The band-pass filter is controlled by a feedback loop connected to the output of the band-pass filter. From a signal-to-noise ratio point of view this structure is less ideal, a fact which has also been pointed out and remedied by the inventor by means of adding complexity in form of a frequency dividing network to the system. Furthermore, the use of a feedback loop for control inherently increases the risk of overload due to overshoot in the control loop. Finally, the system does not compensate for the lack of bass impression at high levels where the compensation filter is effectively inactive.
The method described by Magrath includes a fixed low-pass filter, of typically second order, together with a very simple compressor, and addition of the low-pass filtered signal to the input signal. The resulting overall frequency response of the system consists of 1) a low frequency range with essentially flat response, corresponding to the pass-band (pole pair frequency) of the low-pass filter, 2) a transition band of a width depending of the amount of gain applied in the compressor, and 3) a high frequency range with essentially flat frequency response. Effectively, the system response comprises a fixed pole pair frequency and a variable zero pair frequency. This type of frequency response is not well suited to mirror the frequency response of the typical loudspeaker, which has one or two pole pairs at essentially fixed frequency and Q-value.
In order to be able to create an impression of low-pitched bass signals without having to reproduce the actual low-frequency components, methods which rely on the principle of virtual pitch have been developed. Two of these methods are described in the U.S. Pat. No. 5,930,373 (Shashoua et al., of K. S. Waves Ltd.) and U.S. Pat. No. 6,134,330 (De Poortere et al., of Philips). Both methods generate harmonic distortion components from the lowest input frequencies by means of a non-linear transfer function. These harmonic components carry the information of the low pitch even if the fundamental frequency component cannot be reproduced by the loudspeaker. A disadvantage of using just the harmonic components to create the impression of a low pitch is that the perceived timbre is changed from that of the original signal. A disadvantage of using a non-linear transfer function is that not only harmonic components are generated but also intermodulation distortion, which is undesirable.
Another method, which also relies on the principle of virtual pitch, is described in U.S. Pat. No. 6,285,767 (Klayman, of SRS Labs, Inc.). One aspect of U.S. Pat. No. 6,285,767 comprises of a level-dependent boost of a fixed frequency range. The frequency range is set by one or more band-pass filters in parallel. The purpose of the band-pass filters is to increase the gain of low frequency harmonic components already present in the signal. Although the harmonic components do carry the information of low pitch, the timbre is undesirably changed compared to the input signal, especially when emphasizing a fixed frequency range.
It is an object of the present invention to overcome the shortcomings and disadvantages of the methods and systems described in e.g. U.S. Pat. No. 5,481,617, U.S. Pat. No. 5,930,373, U.S. Pat. No. 6,134,330, U.S. Pat. No. 6,285,767, and/or US 2004/0022400.